That's what MS-Word says, but they often get it wrong. The rule is Who if it is the subject of the action, and Whom if it is the object of the action. But I'm not sure what the action is, "to choose" or "would be."

Who"Whom", as an indirect, takes a preposition.We write "for whom", "by whom", "with whom", etc. If you ever wonder, try adding a preposition to your question.So: "choose who would be the customers"but "choose for WHOM you are buying these things."

Aristoshark saidWho"Whom", as an indirect, takes a preposition.We write "for whom", "by whom", "with whom", etc. If you ever wonder, try adding a preposition to your question.So: "choose who would be the customers"but "choose for WHOM you are buying these things."

This.

Whom is used instead of who as the object of a verb or preposition:1) I met a friend with whom I went to school.2) I met a friend who went to school with me.